QmlPreview: Protect QQmlPreviewFileLoader::load with another mutex

We can concurrently load multiple files from different threads, but the
loader can only handle one such request at a time.

Fixes: QTBUG-95825
Pick-to: 6.2
Change-Id: I2b94d55f2cf0da6e84dabc47df5699cc57b903a6
Reviewed-by: Maximilian Goldstein <max.goldstein@qt.io>
Reviewed-by: Knud Dollereder <knud.dollereder@qt.io>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileengine.cpp b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileengine.cpp
index 5e78539155..e11b8c9776 100644
--- a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileengine.cpp
+++ b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileengine.cpp
@@ -398,6 +398,14 @@ bool QQmlPreviewFileEngine::supportsExtension(Extension extension) const
 
 void QQmlPreviewFileEngine::load() const
 {
+    // We can get here from different threads on different instances of QQmlPreviewFileEngine.
+    // However, there is only one loader per QQmlPreviewFileEngineHandler and it is not thread-safe.
+    // Its content mutex doesn't help us here because we explicitly wait on it in load(), which
+    // causes it to be released. Therefore, lock the load mutex first.
+    // This doesn't cause any deadlocks because the only thread that wakes the loader on the content
+    // mutex never calls load(). It's the QML debug server thread that handles the debug protocol.
+    QMutexLocker loadLocker(m_loader->loadMutex());
+
     m_result = m_loader->load(m_absolute);
     switch (m_result) {
     case QQmlPreviewFileLoader::File: